Radio detection and ranging (RADAR) has been known since the Second World War. This technique comprises illuminating a target with electromagnetic radiation an receiving backscattered signals from the target at a receiver. From the phase and amplitude information contained in the backscattered signal the target's characteristics can be determined. Due to the nature of electromagnetic propagation and the distances involved from the receiver transmitter set to the target, the resolution of received images has generally been of low quality.
In order to improve the resolution of the images, RADAR transmitter sets have developed with an eye toward increasing the aperture size. With large apertures sizes and wavelengths in the microwave region, the wavelength to aperture size ratio is small enough so that the degree of resolution is within a few radians. Typically, the resolution for wavelengths in the optical region is between 10.sup.-4 to 10.sup.-5 radians. However, to achieve the same degree of resolution for wavelengths in the microwave region it would be necessary to have aperture sizes between 100 meters and 30 kilometers.
One way of overcoming the problem of attaining large aperture sizes in microwave imaging systems is to utilize a phased array. Phased arrays are well known in the RADAR art. They are usually distributed randomly or in a pre-determined pattern such that the properties of the backscattered radiation can be described only statistically. Phased arrays allow the designer to achieve resolutions such that images formed are similar to those formed with optical cameras. Therefore, high resolution image formation with a phased array has been dubbed by persons having ordinary skill in the art as the "radio camera". The aperture of a "radio camera" is a highly thinned array in which the individual elements are separated from one another and distributed over an area which can be as large as 1,000 square kilometers, or larger if the wavelengths increase.
Since the elements of a radio camera are distributed over such a large area, the design must circumvent the requirement that the array surface be planar or otherwise have a fixed spatial orientation. The system design must be sufficiently robust to permit the coordinates of each individual element to be arbitrarily chosen and even possibly to vary in time. Since the scattered images from a target will reach each element of the array at different times it becomes necessary to design a low order intelligence into the system which will allow beam forming and scanning to proceed in the absence of accurate knowledge of each element's position as a function of time. A low order intelligence produces the property of self-coherence sometimes called self-phase, or self focus. This method of self-coherence is known to those of ordinary skill in the art as Adaptive Beam Forming (ABF). See Steinberg, Microwave Imaging With Large Antenna Arrays, pp 135-138, John Wiley & Sons (1983).
In conventional systems such as ordinary optical, microwave or RADAR systems, self-coherence is inherent. That is, the conventional telescope mirror or microwave dish has an accurately shaped reflecting surface that focuses energy to a point. The surface is shaped so that rays that leave a distant source simultaneously arrive at the surface and are reflected to a common point called the focus or focal point. The shaped mirror or dish equalizes the travel times of all rays from source to surface to focal point . Lenses accomplish the same result by varying their thicknesses with position, differentially, so that light rays passing through them are provided with a single focal point.
Because a phased array is spatially distributed, it cannot depend on a surface shape to time align the signal passing through it. Instead, a phased array must have electrical circuits to delay the signal differentially from the different parts of the array so that simultaneity is achieved. Persons of ordinary skill in the art are well aware of techniques to achieve this electrical phase alignment known as Adaptive Beam Forming.
The radio camera only functions effectively in receiving signals from scattered targets. The adaptive beam forming technique with a radio camera allows high resolution images from scattered radiation to be produced. However, there is still a long felt need in the art for systems that can identify unknown airborne targets by first illuminating the target with, for example radiation from the ground to be scattered back toward a phased array. Such a system has not yet been achieved so as to produce satisfactory images adequate for high resolution target identification.
Synthetic Aperture Radar (SAR) has been known in the art since the middle 1950's. See Steinberg, Microwave Imaging With Large Antenna Arrays at p. 11. As evidenced by its name, the aperture in SAR is not the standard aperture understood by the RADAR art. SAR is created by putting a transmitter on a moveable object such as an airplane and allowing the airplane to travel a distance L equal to its velocity multiplied by the time. As the airplane travels a distance, L, an on-board RADAR transmitter pulses at some extremely short time interval. The effective aperture size is the velocity V the airplane travels over the time period T rather than the small fixed mechanical aperture size of the RADAR itself. L is the synthetic aperture. This creates a larger aperture than the mechanical aperture of the transmitter. Therefore, the wavelength-to-aperture ratio, which defines the effective resolution of the system is dramatically decreased by the larger aperture size.
SAR, in a non-accelerating airplane, transmits a sequence of pulses at equal intervals and receives their echoes from equally spaced positions along the aircraft's flight path. By combining the received signals coherently, the system synthesizes the equivalent of a large antennae array given that the aircraft speed is V, the period between pulses is T and the number of coherently integrated pulses is N. The distance the aircraft flies during this time interval is L=NTV which is the length of the synthetic aperture. However, deviations from non-accelerating flight, due for example to wind gusts, distort the synthetic aperture and require correction. This has been done with measurements made within the airplane with pendulums, accelerometers or other inertial devices that note the instantaneous acceleration of the platform from which the instantaneous deflection from straight line flight can be calculated. These measurements lead to phase corrections made in the receiver local oscillator or in the resulting data set to compensate for the aperture distortions. This requires that the aircraft housing the synthetic aperture RADAR is "cooperative" so that acceleration errors can be transmitted to ground stations to correct for the phase distortions. SAR is a viable system for this reason since phase and amplitude correction data is readily available from cooperative aircraft which function as the transmitter platform for the SAR.
Real world applications of RADAR require identification of airborne targets which are unknown and possibly hostile. A ground-based transmitter set, or one set in an airborne platform having positional compensation devices, is required to achieve this purpose. High resolution images are an absolute requirement for this system in order to efficiently facilitate target imaging and identification. In the SAR system, the transmitter is located on board the airborne platform which images motionless ground target. Generally, the SAR system is inadequate to perform target imaging and identification of hostile airborne targets. Therefore, a system, preferably ground-based, which can produce a highly resolved image of an accelerating airborne target is desired. Such a system has not heretofore been developed to satisfy this need.
There is a long-felt need for target imaging and identification of airborne targets from, preferably, ground-based systems. Inverse synthetic aperture RADAR (ISAR) has been known for many years to those with ordinary skill in the art as a method for target imaging and identification of airborne targets. In ISAR it is necessary to achieve a large aperture size, as with SAR, in order to achieve the resolution desired. In ISAR, a moving target is illuminated as it travels a distance L over a time T by a the ISAR system. In a preferred embodiment of the ISAR system, the RADAR is on the ground and illuminates a flying target. Therefore, the target's motion is used to synthesize an extended aperture which can then image the target. The successive directions of arrival of the pulses from the reflected radiation are generally line of sight vectors from the target to the receiving set. From the frame of reference of the airborne target the apparent sources of these radiated pulses come from a succession of nearly equally space positions on the ground. This set of positions forms a synthetic aperture. The length of the aperture is as in SAR, L=NTV.
Although the principal of ISAR is thirty years old, ISAR is intrinsically more difficult to implement than SAR because the RADAR and the airborne platform are not co-located. Co-located RADAR sets are RADARs in which the transmitter is contained within the moving platform. As a consequence, the ISAR system has no means for obtaining the mechanical measurements of instantaneous target aircraft acceleration for correction of the RADAR data. The hostile airborne target is buffeted by winds, may have internal acceleration changes and is also flexed and distorted by its travel through the air at high speeds. These changes in acceleration are not measurable on a hostile aircraft and therefore cannot be transmitted to the ISAR ground station for phase correction. Therefore, while ISAR is a concept which has been known for many years, it has been virtually impossible to obtain a highly resolved target image using ISAR because of the acceleration errors which produce phase distortions that are intolerable to high resolution images.
There has been a long felt need in the RADAR art for high resolution images for identification of airborne targets. No system exists today which fulfills this long felt need since phase and amplitude errors which are not correctable within a ground RADAR transmitter/receiver set exist in virtually all airborne targets. No single RADAR imaging system has been known heretofore capable of producing highly resolved images which are adaptable for imaging and identification of airborne targets. Many others have attempted to solve this problem but have failed.